Recently, organic EL display panels are gaining popularity as display devices. A typical organic EL display panel includes a substrate and organic EL elements disposed on the substrate. An organic EL display panel realizes high visibility and high shock resistance, since the organic EL elements included therein have high visibility for being self-luminescent and have high shock resistance for having a fully solid-state structure.
An organic EL element is a current-driven light-emission element, and typically includes an electrode pair composed of an anode and a cathode, and a plurality of organic EL functional layers that are layered between the anode and the cathode. The organic EL functional layers include a light-emission layer that emits light by utilizing electroluminescence occurring when carriers (holes and electrons) recombine therein.
In the process of manufacturing an organic EL display panel, there are cases where an organic EL functional layer, an electrode, etc., in an organic EL element, is formed to have a locally uneven surface. Such a locally uneven surface may be formed, for example, as a result of a foreign particle of extremely small size, such as dust, adhering to the surface of the organic EL functional layer, the electrode, etc., or as a result of the presence of a defect in the resist used for forming the organic EL functional layer, the electrode, etc. Such a locally uneven surface of the organic EL functional layer, the electrode, etc., may result in an organic EL functional layer formed above the locally uneven surface having an area with uneven film thickness. Further, such a locally uneven surface may even result in a local absence in the organic EL functional layer formed thereabove. The forming of such an area having uneven film thickness and/or such a local absence in the organic EL functional layer may result in short-circuiting between the anode and the cathode, which is a cause of non-light emission of the light-emission layer. In the technical field to which the present invention belongs, an organic EL element whose light-emission layer does not emit light even when the organic EL display panel is driven is commonly referred to as a dark spot, a dead pixel, etc. As such, in the present disclosure, an organic EL element whose light-emission layer does not emit light (i.e., a non light-emitting pixel in an organic EL display panel) is referred to as a dark spot.
Here, it should be noted that dark spots also include, in addition to organic EL elements whose light-emission layers do not emit light permanently from the beginning, organic EL elements in which the short-circuiting only occurs from time to time (non-permanently) and organic EL elements that emit light normally in the beginning but do not emit light after a certain period of time elapses. In particular, when dark spots occur in an organic EL display panel after the organic EL display panel is shipped as a product, the user of such an organic EL display panels suffer from inconvenience. As such, various measures are being taken to detect and repair such potential dark spots in organic EL display panels before shipment.
For example, Patent Literature 1 discloses one method of making a potential dark spot detectable. According to the method disclosed by Patent Literature 1, migration of aluminum included in a cathode takes place when a predetermined reverse bias voltage is applied to an organic EL element that has a defective portion. This results in breakdown occurring in the defective portion and the organic EL element being put in a permanent short-circuit state. Thus, the organic EL element having the defective portion, which is a potential dark spot, is made detectable as a dark spot.
Patent Literature 2 discloses one method of repairing a dark spot. According to the method disclosed by Patent Literature 2, when a reverse bias voltage is applied to an organic EL element that has a defective portion, current flows in a concentrated manner through the defective portion. This results in the temperature of the defective portion rising, which brings about some form of change in the defective portion that increases the electrical resistance of the defective portion to a point where the defective portion no longer conducts current. As examples of such changes that the defective portion undergoes due to the application of the reverse bias voltage and the resultant rise of temperature, Patent Literature 2 discloses the defective portion burning out, the defective portion transitioning into the gas phase, and the defective portion transforming into an electrically-insulative body through oxidization/carbonization.